feat: add Antigravity CLI (agy) support

Antigravity (Google's `agy` CLI) installs the existing Superpowers plugin
directly:

    agy plugin install https://github.com/obra/superpowers

agy imports the bundled skills and runs the plugin's SessionStart hook, so
using-superpowers bootstraps from the first message — verified on agy 1.0.3:
a fresh session given "Let's make a react todo list" auto-triggers the
brainstorming skill instead of writing code. agy discovers skills natively
and, having no Skill tool, loads them by reading SKILL.md with view_file.

No scaffold, installer, or generated context file is needed. This adds only:

- README.md: an Antigravity install section + Quickstart link
- skills/using-superpowers/SKILL.md: reference to the agy tool mapping
- skills/using-superpowers/references/antigravity-tools.md: action->tool
  mapping for agy (view_file, write_to_file, invoke_subagent, manage_task,
  and skill loading via view_file on SKILL.md)
- tests/antigravity/: structural test for the tool mapping, mirroring
  tests/pi/

README.md

@@ -4,7 +4,7 @@ Superpowers is a complete software development methodology for your coding agent
 
 ## Quickstart
 
-Give your agent Superpowers: [Claude Code](#claude-code), [Codex App](#codex-app), [Codex CLI](#codex-cli), [Cursor](#cursor), [Factory Droid](#factory-droid), [Gemini CLI](#gemini-cli), [GitHub Copilot CLI](#github-copilot-cli), [OpenCode](#opencode), [Pi](#pi).
+Give your agent Superpowers: [Claude Code](#claude-code), [Antigravity](#antigravity), [Codex App](#codex-app), [Codex CLI](#codex-cli), [Cursor](#cursor), [Factory Droid](#factory-droid), [Gemini CLI](#gemini-cli), [GitHub Copilot CLI](#github-copilot-cli), [OpenCode](#opencode), [Pi](#pi).
 
 ## How it works
 
@@ -60,6 +60,17 @@ The Superpowers marketplace provides Superpowers and some other related plugins
   /plugin install superpowers@superpowers-marketplace
   ```
 
+### Antigravity
+
+Install Superpowers as a plugin from this repository:
+
+```bash
+agy plugin install https://github.com/obra/superpowers
+```
+
+Antigravity runs the plugin's session-start hook, so Superpowers is active from
+the first message. Reinstall with the same command to update.
+
 ### Codex App
 
 Superpowers is available via the [official Codex plugin marketplace](https://github.com/openai/plugins).